The present disclosure relates to a system and method for determining the compressive strength of materials. Compression test systems have been commonly used to characterize or measure the compressive strength of materials, including mined materials such as taconite, agglomerated products, and other materials commonly provided in a pelletized form. The compressive strength of such pellets is important to ensure the pellets have sufficient strength to withstand transportation or other process operations. For example, during transportation substantial quantities of material may be placed into the hold of a ship or into containers for rail or truck transportation and, as the amount of material increases, substantial compressive force is applied to the pellets. Various systems have been developed to characterize the compressive strength or crushing strength of pellets, and requirements such as ISO 4700 have been developed to standardize these measurements.
Prior compression test systems have typically had limited throughput due to the time required to test a pellet. Test specifications or standards often impose a limit on the operating speed of the compressive test system, and in some cases, the operating speed has been limited to 10 to 20 millimeters per minute. To accommodate variation in pellet size, many prior test systems positioned the ram at a starting location sufficiently removed from the platen to accommodate the largest pellet expected to be tested. As most pellets have a diameter less than the diameter of the largest pellet to be tested, this prior approach to positioning the ram has resulted in increased test times as the ram is required to travel a greater distance for each pellet.
Additionally, prior test systems were often unable to accurately capture the maximum compressive strength of a pellet under test due to limitations of the test system. Substantial user intervention has often been required limiting the efficiency and throughput of prior compression test systems. To improve accuracy, the measurement device of prior test systems have often been tested by stacking known weights on the test system and verifying the accuracy of the measurement. The operating range of compression test systems often extends to several hundred pounds, and in some cases, to over 1,000 pounds. As a consequence, positioning this amount of weight on the test system has been cumbersome and has lead to complicated maintenance and calibration procedures for prior test systems.
In view of the limitations of prior systems, there remains a need for a system and method for determining the compressive strength of pellets that provide for increased efficiency and reduced costs. There also remains a need for a compression test system requiring less user intervention and allowing the testing of larger samples of pelletized materials. There also remains a need for a simplified maintenance and calibration system and method to improve the usability and maintainability of the compression test system.
Presently disclosed is a system for determining the compressive strength of pellets comprising a moveable feed tray adapted to support a plurality of pellets to be tested, a platen positioned below the moveable feed tray and opposite a ram capable of applying a compressive force to a pellet on the platen, a drive motor engaging the feed tray to position a pellet between the platen and the ram to permit compression testing of the pellet, and a compressive force monitoring system including a force sensor, where the compressive force monitoring system is capable of providing a first signal corresponding to the force applied by the ram to a pellet on the platen and capable of determining the maximum compressive force applied by the ram to the pellet on the platen.
In the system for determining the compressive strength of pellets, the ram may be hydraulically powered, and the moveable feed tray may be a rotating feed wheel. The feed wheel may include a plurality of apertures spaced along the circumference of the feed wheel, each aperture being adapted to support a pellet. In one embodiment, the feed wheel may have 100 or more apertures. The force sensor may comprise a load cell. The compressive force monitoring system may include a signal processor capable of determining the maximum compressive force applied to the pellet on the platen. A signal processor may determine the maximum compressive force applied to the pellet on the platen by analog peak capture. The moveable feed tray may further comprise a position indicator, and the position indicator may have a plurality of position indicating apertures, each position indicating aperture corresponding to a pellet support aperture of the feed tray.
The system for determining the compressive strength of pellets may further comprise a position detection system configured to detect the position of the feed tray, the position detection system having a light source on one side of the feed tray position indicator and an optical sensor opposite the light source on the other side of the feed tray position indicator, where the optical sensor receives light from the light sensor passing through a position indicating aperture when the feed tray is in a desired position.
Also disclosed is a method for determining the compressive strength of pellets that comprises providing a feed tray adapted to support a plurality of pellets to be tested, positioning a ram at a first position above a platen to apply a compressive force to a pellet on the platen, advancing the feed tray to position a pellet at a test position between the ram and the platen, detecting the non-arrival of the pellet at the test position, retracting the ram from the first position, advancing the feed tray to position the pellet beneath the ram, and operating the ram to determine the compressive strength of the pellet.
The step of detecting the non-arrival of a pellet may further comprise starting the advance of the feed tray and measuring the time the feed tray has been advancing to position a pellet at the test position between the ram and the platen, and identifying the non-arrival of the pellet at the test position when the measured time exceeds a predetermined time. The step of advancing the feed tray may further comprise advancing the feed tray to position a pellet at a test position between the ram and the platen until the ram in the first position interferes with an advancing pellet supported by the feed tray impeding the advance of the feed tray. The step of retracting the ram from the first position may comprise retracting the ram from the first position such that the ram does not interfere with the advancing pellet supported by the feed tray.
A method for determining the compressive strength of pellets may further comprise retracting the ram from the first position by a predetermined distance, advancing the feed tray to position the advancing pellet at the test position, detecting the non-arrival of the pellet at the test position, and repeating until the ram is sufficiently retracted from the first position such that the ram does not interfere with the pellet supported by the feed tray.
A method of calibrating a compression test system is also disclosed that comprises providing a compression test system having a platen engaging a first load cell and a ram opposite the platen, removing the platen of the compression test system, providing a second load cell with a known measurement accuracy above the first load cell, providing a calibration load applicator and a thrust bearing between the second load cell and the ram, the calibration load applicator being extendable to apply a calibration load to the compression test system, and calibrating the first load cell by comparing the force measured by the second load cell with the force measured by the first load cell. The calibration load applicator may be a threaded connector, such as a bolt or a modified platen.
A calibration system for calibrating a load cell of a compression test system is also disclosed, where the calibration system comprises a master load cell positioned above the load cell of the compression test system, and a calibration load applicator and a thrust bearing positioned between the master load cell and a ram of the compression test system, where the calibration load applicator is extendable to apply a calibration load to the compression test system.